Reset command info cache only if it is null

Since, helper is a singleton that persists through multiple
pssa invocation, in a given powershell session, this
helps us reuse the commandinfo cache created during
the first pssa invocation. Results in significant perf
improvement for subsequent pssa invocations.

Author: Kapil Borle

Engine/Helper.cs

@@ -150,7 +150,10 @@ public void Initialize()
             KeywordBlockDictionary = new Dictionary<String, List<Tuple<int, int>>>(StringComparer.OrdinalIgnoreCase);
             VariableAnalysisDictionary = new Dictionary<Ast, VariableAnalysis>();
             ruleArguments = new Dictionary<string, Dictionary<string, object>>(StringComparer.OrdinalIgnoreCase);
-            commandInfoCache = new Dictionary<string, CommandInfo>(StringComparer.OrdinalIgnoreCase);
+            if (commandInfoCache == null)
+            {
+                commandInfoCache = new Dictionary<string, CommandInfo>(StringComparer.OrdinalIgnoreCase);
+            }
 
             IEnumerable<CommandInfo> aliases = this.invokeCommand.GetCommands("*", CommandTypes.Alias, true);